In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. The reverse of disproportionation, such as when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as synproportionation.
More generally, the term can be applied to any desymmetrizing reaction where two molecules of one type react to give one each of two different types:
2A -> A' + A''
This expanded definition is not limited to redox reactions, but also includes some molecular autoionization reactions, such as the self-ionization of water.
The first disproportionation reaction to be studied in detail was:
2 Sn^2+ -> Sn^4+ + Sn
This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it söndring.
Mercury(I) chloride disproportionates upon UV-irradiation:
Hg2Cl2 -> HgCl2 + Hg
Phosphorous acid disproportionates upon heating to give phosphoric acid and phosphine:
4 H3PO3 -> 3 H3PO4 + PH3
Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
2 HCO3- -> CO3^{2}- + H2CO3
The oxidation numbers remain constant in this acid-base reaction.
Another variant on disproportionation is radical disproportionation, in which two radicals form an alkene and an alkane.
{2CH3-\underset{^\bullet}CH2 -> {H2C=CH2} + H3C-CH3}
Disproportionation of sulfur intermediates by microorganisms are widely observed in sediments.
4 S^0 + 4 H2O -> 3 H2S + SO4^{2}- + 2 H+
3 S^0 + 2 FeOOH -> SO4^{2}- + 2FeS + 2 H+
4 SO3^{2}- + 2 H+ -> H2S + SO4^{2}-
Chlorine gas reacts with dilute sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows:3Cl2 + 6 OH- -> 5 Cl- + ClO3- + 3 H2O
The chlorine reactant is in oxidation state 0.
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Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching pulp to produce high brightness paper. Industrially, sodium chlorate is produced by the electrolysis of concentrated sodium chloride solutions. All other processes are obsolete.
Thiosulfate (IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e.g. sodium thiosulfate . Thiosulfate also refers to the esters of thiosulfuric acid, e.g. O,S-dimethyl thiosulfate . The prefix thio- indicates that the thiosulfate is a sulfate with one oxygen replaced by sulfur. Thiosulfate is tetrahedral at the central S atom.
Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group.
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